The invention is based on a pressure moderator for anti-skid hydraulic brake systems (ABS systems). Hydraulic brake systems, for instance also having multicircuit tandem master brake cylinders, are known (German Offenlegungsschrift No. 27 23 734); it is also known to provide such brake systems with anti-skid braking (ABS) functions, for example incorporating electrically actuatable multi-position magnetic valves, which realize the desired and necessary ABS control functions, in the outgoing brake pressure lines leading to the wheel brake cylinders, a pressure drop operation is such that the particular pressure line from the master brake cylinder to the associated wheel brake cylinders is at least interrupted and pressure fluid delivered to the return system. If a renewed pressure rise is desired, then communication with the master brake cylinder is reestablished, along with arbitrary pressure/time courses, and with the option of realizing functions of keeping pressure unchanged, without either a rise or a drop. Realizing ABS functions in this way, with the aid of electrically actuatable multi-position magnetic valves in the main brake lines, can be problematic, because brake medium or brake fluid that is under pressure is practically "lost" and must be pumped back up again later, in the area of a pressure supply apparatus having a pump, a pressure storage tank and appropriate pressure switches, to the high pressure that is required for the braking process. Furthermore, it is also necessary to design such systems, which effect an interruption of the brake lines and an outflow of the pressure fluid in them, very reliably and in a very fail-safe manner, so that if a failure occurs there will be no danger that too much high-pressure brake fluid will be drained out.
In the field of realizing ABS functions, it is also known (U.S. Pat. Nos. 3,690,736 and 3,371,979) to attach a parallel branch in the lines leading from the master brake cylinder to the various wheel brake cylinders, and to connect this branch with a chamber which is variable, by means of an electrically actuatable cylinder-piston assembly, in accordance with the triggering of an actuating coil counter to the pressure of a spring, so that a volume of pressure can be removed from and then returned to the brake line again; this is known as the basic plunger principle. In this case, although the brake circuit again remains closed, means are still required for interrupting the further delivery of highpressure brake pressure fluid from the master brake cylinder during ABS functions, so that the removal of pressure fluid can become at all operative. This interruption can also be made possible by providing (U.S. Pat. No. 3,690,736) that the retreating plunger simultaneously liberates a ball valve so as to close the master brake cylinder feed line. However, if the system fails at this point, then it is no longer possible to generate any braking pressure at all from the master brake cylinder, that is, by actuating the brake pedal.
In a brake actuating apparatus for a vehicle brake system, it is known (European Pat. No. 0025714) for at least one reservoir and one electrically actuatable valve, and preferably a pump for filling the reservoir as well, to be incorporated directly into the brake actuating apparatus, embodied by a caliper, for instance for a disk brake. By suitable miniaturization, the result is a combination brake pressure modulator and caliper, and an additional function of this combination is also to detect any possible locking or skidding states of a particular braked wheel and then to actuate the integrated valve in such a way as to prevent locking or skidding (ABS).